You are conflating structures used internally to implement a relational database, and structures exposed as a first-class building block for your own systems.

You are conflating structures used internally to implement a relational database, and structures exposed as a first-class building block for your own systems. Think of Redis a a shared, in-memory, well-tested implementation of basic data structure primitives that takes care of a lot of the fiddly nonsense around them without you having to do the work. Postgres is awesome, but it's targeted at a different use case and has different characteristics; it's overkill for something like a shared key-value store, for example.